The present invention relates to a method for correcting gradation of output data for reproducing a picture image so as to compensate for the drop in resolving power of gradation of the output data, or the smoothness of gradation of the reproduction picture, for use in a picture digital processing system wherein picture data is processed by using one or a plurality of lookup table memories aligned in series.
In conventional picture digital processing using lookup table memories aligned in series, especially including at least one lookup table memory having non-linear input-output characteristics, the resolving power of the gradation of output data deteriorates, with a result that gradation of the reproduced picture deteriorates.
There are shown in FIGS. 1 and 2 input-output characteristics curves A and B of conventional lookup table memories which are aligned in series and are used for a conventional picture digital processing system.
There are shown in FIGS. 3 and 4 parts of the input-output characteristics curves A and B of FIGS. 1 and 2, which are enlarged to one least significant bit (LSB) unit scale.
First, when input data X.sub.0, X.sub.1, X.sub.2, . . are input to the lookup table memory of FIG. 3 at t.sub.0, t.sub.1, t.sub.2, . . , as shown in FIG. 5, the table outputs data Y.sub.0, Y.sub.1, Y.sub.2, . . , consecutively. Then, when the output data Y.sub.0, Y.sub.1, Y.sub.2, . . are input to the lookup table memory of FIG. 4, it outputs data Z.sub.1, Z.sub.4, Z.sub.7, . . , finally. However, as shown in FIG. 6, the resultant output data possess far lower resolving power than the original input data.